1. Field of the Invention
The present invention relates to soluble polymer-supported chiral phosphines and, more particularly, relates to soluble polyester-supported chiral phosplines. The polyester-supported phosphine ligands have been used in the preparation of rhodium and ruthenium catalysts which are recyclable and useful in asymmetric catalysis.
2. Description of the Related Art
Polymer-supported catalysts have been attracting much attention due to their easy separation from the reaction mixture and the recyclability of the catalysts. The traditional polymer-supported catalysts used insoluble polymers or inorganic materials as support. The heterogenized catalysts can be separated easily from the reaction mixture via filtration or centrifugation at the end of catalytic reaction. However, the insoluble polymer-supported catalysts usually gave very low catalytic activity and enantioselectivity as compared to the corresponding free catalysts. The main reason is the restriction of the polymer matrix which results in the limited mobility and accessibility of the active sites. For example (see Achiwa, K. Chem. Lett., 1978, 905), a cross-linked polyalkyl methacrylate-supported Rh(BPPM) catalyst was found to catalyze the asymmetric hydrogenation of (Z)-acetamido-cinnamic acid to N-acetylphenylalanine in far lower e.e. (23%) as compared to the free Rh(BPPM) catalyst which gave 91% e.e.
Taking the above circumstances into consideration, extensive studies have been conducted in improving the catalytic activity and enantioselectivity of the polymer-supported catalysts. For example (see Nagel, U. et. al., Chem. Ber., 1996, 129, 815), the chiral phosphine DEGPHOS was supported on cross-linked polystyrene via polyethylene oxide chains which acted as "spacer" to enhance the mobility and accessibility of the catalytic species. The resulting polymer-supported bis(phosphane) rhodium catalyst showed about the same catalytic activity and enantioselectivity as the corresponding free catalyst in the asymmetric hydrogenation of dehydroamino acids. However, after one recycle, this polymer-supported catalyst abruptly lost activity. Another example (see Wan, K. T. et. al., Nature, 1994, 370, 449) is the immobilization of the water soluble organometallic complex, [Ru(BINAP-4SO.sub.3 Na)(benzene)Cl.sub.2 ], in a thin ethylene glycol film on a high-surface area hydrophilic support such as silica or control-pore glass. The resulting catalyst was used to catalyze the hydrogenation of dehydronaproxen to give naproxen with similar enantioselectivity as the corresponding free Ru(BINAP) catalyst, but the catalytic activity was low (trim-over-frequencies were 40.7 hr.sup.-1 vs 131.0 hr.sup.-1).
Insoluble polymer-supported catalysts usually have low mechanical strength which may result in the change of the polymer structure, e.g. polystyrene beads collapse and shrink in polar solvents during catalytic reactions. The poor stability of the polymer-supported catalyst also results in the "leaching" of the noble metal from the support to the reaction solution. For these reasons the insoluble polymer-supported catalysts are not commercially attractive.
Another less known approach is to use a soluble polymer as the catalyst support. Recently, cinchona alkaloid type ligands were anchored on a soluble polyethylene glycol which showed similar catalytic activity and selectivity as compared to the corresponding free catalysts in the osmium-catalyzed asymmetric dihydroxylation of unfunctional olefins (see Han, J. and Janda, K. D., J. Am. Chem. Soc. 1996, 118, 7632; and Bolm C. and Gerlach, A., Angew. Chem. Int. Ed. Engl. 1997,36, 741). However, the separation of the polymer-supported ligands from the reaction mixtures is inconvenient and needed the use of a large amount of ether which is unsafe in industrial operation.